1. Field of the Invention
The disclosures herein generally relate to a memory circuit, and particularly relate to a memory circuit having a test circuit to check an amount of current flowing through a cell transistor.
2. Description of the Related Art
Conventionally, a nonvolatile memory cell requires a special structure such as a floating gate or a special material such as a ferroelectric material or ferromagnetic material for the purpose of achieving nonvolatile data retention. There is a new type of nonvolatile semiconductor memory device called PermSRAM, which uses a MIS (metal-insulating film-semiconductor) transistor as a nonvolatile memory cell (i.e., the basic unit of data storage). The MIS transistor used as a nonvolatile memory cell in PermSRAM has the same structure as ordinary MIS transistors used for conventional transistor functions (e.g., switching function), and do not require a special structure or a special material as described above. The absence of such a special structure and special material offers an advantage in cost reduction. PermSRAM was initially disclosed in PCT/JP2003/016143, which was filed on Dec. 17, 2003, the entire contents of which are hereby incorporated by reference.
The MIS transistor used as a nonvolatile memory cell in PermSRAM is configured to experience an irreversible hot-carrier effect on purpose for storage of one-bit data. Here, the irreversible hot-carrier effect refers to the injection of carriers into the oxide film (insulating film) and/or sidewalls, which causes a change in the transistor's threshold voltage. Whether the threshold voltage has been changed due to a hot-carrier effect represents one-bit data “0” or “1”. Such a change in the threshold voltage may be detected by sensing a difference in ON current between the nonvolatile-memory-cell MIS transistor and a reference MIS transistor by using a sensing circuit such as a latch circuit.
When a nonvolatile memory device having nonvolatile memory cells as described above is manufactured, there is a need to conduct a test to ensure that the memory cells perform properly as designed. Testing, however, cannot be performed by use of a conventional test technique, which checks memory cells by writing data to and reading data from these cells. This is because the operation of the above-noted nonvolatile memory cell is founded on an irreversible change of the transistor characteristics. If a test that creates such an irreversible change is actually performed, the memory circuit may no longer be usable.
A test circuit may be provided to compare an amount of current flowing through a nonvolatile memory cell with an amount of current flowing through a test-purpose reference cell. Such a test circuit can determine whether the tested cell transistor can be sufficiently conductive to serve as a memory cell. Occurrence of an excessive amount of current flowing through a cell transistor, however, cannot be detected by this type of test circuit. A cell transistor that allows an excessive amount of current flow is not suitable as a memory cell because an excessively strong hot-carrier effect may be required to sufficiently change an ON current for the purpose of data storage. There may even be a case in which data cannot be properly stored in the cell transistor.
Accordingly, it may be desirable to provide a memory device that can test a cell transistor to check the amount of current flowing through the cell transistor.